## Desert Diary

Math/Mark and Recapture

Counting a population of animals in a big open space like a desert, is
not as simple as standing and counting. Biologists use a mathematical method called
"capture-recapture". Let's say you capture 100 snakes in a desert region.
You carefully put a tag on them, then set them free. Later, you capture a fresh sample
of 100 snakes and find that 20 of them have tags. Under certain assumptions, the
proportion of snakes in our second ("recapture") sample that are tagged
should equal the proportion of snakes from the total desert population we took our
first sample from.

So now let's solve for that unknown total population: 20 out of 100
equals 100 out of what? The answer....: 500 snakes! This capture-recapture technique
doesn't apply only to counting animals. Every 10 years the US Census tries to count
all the country's humans and then the recapture phase is the Post-Enumerative
Survey. I hope this captured your attention, because in the desert, math counts!

Contributor: Lawrence M. Lesser,
Mathematical Sciences, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP
National Public Radio at the University of Texas at El Paso.

## References

1999. Studying Proportions Using the Capture-Recapture Method.
Mathematics Teacher, Mar., pp. 215-218.

### Web Resources

Challenge

On-line
Workshop